The present invention relates to a directional control valve. More specifically, the present invention relates to a directional control valve, such as a four way directional control valve or a two way directional control valve.
For example, in a four way directional control valve, when the two output ports are blocked, the rate of change of the differential pressure between both the output ports relative to the displacement of the spool is referred to as "blocked pressure gain".
In a four way directional control valve having a metering position, since the communication of the ports is switched at the neutral position of the spool, theoretically speaking, the blocked pressure gain must be infinite at the switching position under the zero lap condition.
However, in an actual valve, because there are clearances between the sleeve and the spool, a small amount of working fluid leaks through the clearances, and therefore, the observed pressure gain is finite as illustrated in FIG. 3.
It is usual in commercially available four way control valves that the maximum differential pressure, i.e., 100% pressure, is obtained at a spool stroke between 5 and 20 micron meter.
In the meantime, it has been observed that the pressure gain as illustrated in FIG. 3 is too large for a circuit for chucking a fragile article or for a circuit for actuating simultaneously hydraulic equipment disposed in parallel, for example, in an aircraft, wherein a so called "force fight" may occur easily because a plurality of actuators are disposed. In such a case, a circuit as illustrated in FIG. 4, i.e., cross bleeding, has been often used to lower the blocked pressure gain.
More specifically, two output ports 5 and 6 are communicated with each other via a fixed orifice 22, and bleeding therebetween is permitted.
When the blocked pressure gain is lowered by cross bleeding which is illustrated in FIG. 4, the obtained pressure gain typically draws such a curve as illustrated in FIG. 5.
However, there occur the following problems if a directional control valve having a pressure gain as illustrated in FIG. 5 is used.
Since bleeding of the pressure occurs through the fixed orifice 22 communicating both the output ports 5 and 6, the required maximum pressure cannot be achieved. In other words, in a control system having this directional control valve installed therein, the power of the pressure in the system cannot be fully utilized.
In addition, as is apparent from FIG. 5, since the pressure gain is not linear, the gain nears zero at region where the differential pressure is relatively high. Accordingly, in an actual control system, a steady-state deviation increases beyond an acceptable level under a large load.